Underground is a big problem with it comes to water permeation and flooding.

The other potential problem with a lot of energy systems that use things that have hydrogen in them is running power over the systems for long periods of time can start to leak hydrogen. You'll always want to ensure you have ventilation to ensure whatever is outgassing can escape the system. You'll end up with explosions, hydrogen embrittlement, or interesting corrosion in ways you didn't think were possible.